Power supply apparatus varying an output constant voltage in response to a control signal from a load circuit

ABSTRACT

A power supply apparatus includes an output voltage generator, a reference voltage generator, a voltage divider, and a voltage control circuit. An input voltage is supplied from a direct current power source. The output voltage generator generates a constant output voltage based on the input voltage. The reference voltage generator generates a reference voltage. The voltage divider divides the constant output voltage into a divided voltage in accordance with a voltage dividing ratio variable in response to an externally-input control signal. The voltage control circuit controls the output voltage generator to regulate the constant output voltage such that the divided voltage from the voltage divider is equalized to the reference voltage.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority under 35 USC §119 to Japanese patentapplication No. JPAP 2002-195406 filed Jul. 4, 2002, and Japanese patentapplication No. JPAP 2002-249081, filed Aug. 28, 2002, the entirecontents of which are incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to a power supply apparatus, and moreparticularly to a power supply apparatus capable of varying an outputconstant voltage in response to a control signal from a load circuit.

BACKGROUND OF THE INVENTION

Conventional power supply apparatus use a switching or series regulatoron a power supply side for outputting a predetermined voltage requiredby a load side. As shown in FIG. 1, a background power supply apparatus100 has a feedback configuration to return an output voltage Vo of aDC-to-DC converter, to divide the output voltage Vo with a voltagedivider 101 into a divided voltage Vd, and to compare the dividedvoltage Vd with a predetermined reference voltage Vr.

Due to a recent trend, however, the voltage required by the load sidehas been reduced, and the power supply apparatus is consequentlyrequired to change the output voltage of the switching or seriesregulator. Also, the power supply apparatus itself needs to be changedwhen a required power supply voltage value is changed by, for example,the replacement of a component used in the load side of a system afterthe power supply apparatus is installed into the system.

In view of the foregoing, it is desirable to change a level of an outputconstant voltage in response to a control signal sent from a load side.

SUMMARY OF THE INVENTION

In one example, a novel power supply apparatus includes an inputterminal, an output voltage generator, an output terminal, a referencevoltage generator, a voltage divider, and a voltage control circuit. Theinput terminal is supplied with an input voltage from a direct currentpower source. The output voltage generator is configured to generate aconstant output voltage based on the input voltage. The output terminaloutputs the constant output voltage. The reference voltage generator isconfigured to generate a reference voltage. The voltage divider has anoutput and is configured to divide the constant output voltage into adivided voltage output from the output in accordance with a voltagedividing ratio variable in response to an externally-input controlsignal. The voltage control circuit is configured to control the outputvoltage generator to regulate the constant output voltage such that thedivided voltage from the voltage divider is equalized to the referencevoltage.

The voltage divider may include a first resistor circuit, a first switchcircuit, a second resistor circuit, a second switch circuit, and aswitch control circuit. The first resistor circuit includes a pluralityof resistors connected in series between the output terminal and theoutput point of the voltage divider. The first switch circuit isconfigured to make a short circuit in at least one of the plurality ofresistors included in the first resistor circuit in response to an inputcontrol signal. The second resistor circuit includes a plurality ofresistors. The second switch circuit is configured to connect inparallel at least one of the plurality of resistors included in thesecond resistor circuit between the output point of the voltage dividerand a common ground of the direct current power source in response tothe input control signal. The switch control circuit is configured togenerate the input control signal in response to the externally-inputcontrol signal and to control the first and second switch circuits withthe input control signal to change the voltage dividing ratio.

The voltage divider may include a first resistor circuit, a first switchcircuit, a second resistor circuit, a second switch circuit, and aswitch control circuit. The first resistor circuit includes a pluralityof resistors. The first switch circuit is configured to connect inparallel at least one of the plurality of resistors included in thefirst resistor circuit between the output terminal and the output pointof the voltage divider in response to an input control signal. Thesecond resistor circuit includes a plurality of resistors connected inseries between the output point of the voltage divider and a commonground of the direct current power source. The second switch circuit isconfigured to make a short circuit in at least one of the plurality ofresistors included in the second resistor circuit in response to theinput control signal. The switch control circuit is configured togenerate the input control signal in response to the externally-inputcontrol signal and to control the first and second switch circuits withthe input control signal to change the voltage dividing ratio.

The output voltage generator may include a switching transistorperforming a switching operation for outputting the input voltageapplied by the direct current power source in accordance with a controlsignal from the voltage control circuit, and the voltage control circuitmay include an error amplifier, a control circuit, and a smoothingcircuit. The error amplifier amplifies an error of the divided voltageoutput from the output point of the voltage divider relative to thereference voltage. The control circuit is configured to generate thecontrol signal in accordance with an output signal from the erroramplifier to control the switching operation of the switchingtransistor. The smoothing circuit is configured to smooth an outputsignal from the switching transistor and to output to the outputterminal.

The reference voltage generator, the voltage divider, the erroramplifier, and the control circuit may be integrated into a singleintegrated circuit.

The reference voltage generator, the switching transistor, the voltagedivider, the error amplifier, and the control circuit may be integratedinto a single integrated circuit.

The smoothing circuit may include a transistor which is operated andcontrolled by the control circuit to function as a flywheel diode, andthe transistor, the switching transistor, the voltage divider, the erroramplifier, the smoothing transistor and the control circuit may beintegrated into a single integrated circuit.

The output voltage generator may include an output control transistorcontrolling an output of a current applied by the direct current powersource in accordance with a control signal from the voltage controlcircuit, and the voltage control circuit may include an error amplifiercontrolling an operation of the output control transistor such that thedivided voltage of the voltage divider is equalized to the referencevoltage.

The reference voltage generator, the voltage divider, and the erroramplifier may be integrated into a single integrated circuit.

The reference voltage generator, the voltage divider, the erroramplifier, and the output control transistor may be integrated into asingle integrated circuit.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of the disclosure and many of the attendantadvantages thereof will be readily obtained as the same becomes betterunderstood by reference to the following detailed description whenconsidered in connection with the accompanying drawings, wherein:

FIG. 1 is a circuit diagram of a conventional power supply apparatus;

FIG. 2 is an exemplary circuit diagram of a power supply apparatusaccording to a preferred embodiment of the present invention;

FIG. 3 is an exemplary circuit diagram of a voltage dividing circuitincluded in the power supply apparatus of FIG. 2;

FIG. 4 is an exemplary circuit diagram of the voltage dividing circuitwhen in FIG. 3 is two;

FIG. 5 is an exemplary circuit diagram of a power supply apparatusaccording to another preferred embodiment of the present invention;

FIG. 6 is an exemplary circuit diagram of a power supply apparatusaccording to another preferred embodiment of the present invention;

FIG. 7 is an exemplary circuit diagram of a power supply apparatusaccording to another preferred embodiment of the present invention;

FIG. 8 is an exemplary circuit diagram of a power supply apparatusaccording to another preferred embodiment of the present invention;

FIG. 9 is an exemplary circuit diagram of a voltage dividing circuitincluded in the power supply apparatus of FIG. 7; and

FIG. 10 is an exemplary circuit diagram of the voltage dividing circuitwhen in FIG. 9 is two.

DETAILED DESCRIPTION OF THE INVENTION

In describing preferred embodiments illustrated in the drawings,specific terminology is employed for the sake of clarity. However, thedisclosure of this patent specification is not intended to be limited tothe specific terminology so selected and it is to be understood thateach specific element includes all technical equivalents that operate ina similar manner. Referring now to the drawings, wherein like referencenumerals designate identical or corresponding parts throughout theseveral views, particularly to FIG. 2, a power supply apparatus 1according to a preferred embodiment of the present invention isexplained. The power supply apparatus 1 of FIG. 2 is an exemplary powersupply apparatus provided with a DC-to-DC (direct current to directcurrent) converter including a voltage-step-down-type switchingregulator.

As shown in FIG. 2, the power supply apparatus 1 is connected to adirect current power source 10, and includes a switching transistor 2, asmoothing circuit 3, a reference voltage generator 4, a voltage dividingcircuit 5, an error amplifier 6, and a control circuit 7. The currentpower source 10 includes a battery and the like and applies a voltageVbat to the switching transistor 2 through an input terminal IN of thepower supply apparatus 1. The switching transistor 2 includes aP-channel MOS (metal oxide semiconductor) transistor (hereinafterreferred to as a PMOS transistor) which outputs the voltage Vbat appliedthereto by the direct current power source 10. The smoothing circuit 3smoothes the signal output from the switching transistor 2 and outputsthe signal having a voltage Vo to an output terminal OUT. The referencevoltage generator 4 generates a reference voltage Vr having apredetermined voltage value. The voltage dividing circuit 5 divides thevoltage Vo and outputs a divided voltage Vd. The error amplifier 6amplifies an error of the divided voltage Vd relative to the referencevoltage Vr. The control circuit 7 controls the switching of theswitching transistor 2 in accordance with the signal output from theerror amplifier 6.

The output voltage Vo is divided by the voltage dividing circuit 5, andthe difference between the resultant divided voltage Vd, which isregarded as an error, is amplified by the error amplifier 6. The controlcircuit 7 includes an oscillator for generating, for example, a pulsesignal having a triangular waveform and a comparator. In the controlcircuit 7, the comparator compares the voltage of the signal output fromthe oscillator with the voltage of the signal output from the erroramplifier 6. In accordance with a result of the comparison, thecomparator controls a time that the switching transistor 2 is beingturned on. The signal output from the switching transistor 2 is smoothedand output as the voltage Vo, by the smoothing circuit 3 which includesa flywheel diode D1, an electric coil L1, and a capacitor C1.

The voltage dividing circuit 5 includes a voltage divider 11 and aswitching controller 12. The voltage divider 11 switches a dividingratio in accordance with an input control signal, and accordinglygenerates and outputs the divided voltage Vd. The switching controller12 controls the switching of the dividing ratio of the voltage divider11 in accordance with a voltage switching signal Sc which is externallyinput thereto.

FIG. 3 breaks down the voltage divider 11 of the voltage dividingcircuit 5, and an exemplary operation of the voltage dividing circuit 5is explained with reference to FIG. 3. The voltage divider 11 includes nresistors in series referred to as resistors RA1-RAn, n resistorsreferred to as resistors RB1-RBn, (n−1) PMOS transistors referred to asPMOS transistors QP1-QPn−1, n N-channel MOS transistors referred to asNMOS transistors QN1-QNn, and a capacitor 16. The resistors RA1-RAn areconfigured as a first resistor circuit, the resistors RB1-RBn areconfigured as a second resistor circuit, the PMOS transistors QP1-QPn−1are configured as a first switching circuit, and the NMOS transistorsQN1-QNn are configured as a second switching circuit.

The resistors RA1-RAn are connected in series between the terminal OUTof the power supply apparatus 1 and the output terminal 15. The PMOStransistors QP1-QPn are coupled to the resistors RA1-RAn, respectively,to form n sets of transistor-resistor parallel circuits, which areconnected in series between the output terminal 15 and a ground. Theswitching controller 12 generates control signals SP1-SPn based on thevoltage switching signal Sc, and applies the control signals SP1-SPn−1to gates of the PMOS transistors QP1-QPn−1, respectively.

A capacitor 16 is also connected between the terminal OUT of the powersupply apparatus 1 and the output terminal 15. The NMOS transistorsQN1-QNn are coupled to the resistors RB1-RBn, respectively, to form nsets of transistor-resistor series circuits, which are connected inparallel between the output terminal 15 and a ground. The controlsignals SP1-SPn of the switching controller 12 are also applied to gatesof the PMOS transistors QN1-QNn, respectively.

The switching controller 12 raises one of the control signals SP1-SPn toa predetermined high level (hereinafter referred to as a level H) anddrops the rest down to a predetermined low level (hereinafter referredto as a level L) in accordance with the externally supplied inputvoltage switching signal Sc. For example, when the control signal SPm israised to the level H and the rest of the control signals are lowered tothe level L, the PMOS transistor QPm is selected among the PMOStransistors QP1-QPn−1 to be turned off to become non-conductive and therest of the PMOS transistors are turned on to be conductive, where m isan integer between 1 and n−1 . At the same time, the NMOS transistor QNmis selected among the NMOS transistors QN1-QNn to be turned on to beconductive and the rest of the NMOS transistors are turned off to becomenon-conductive. Thereby, the resistors RAm and RAn are connected inseries between the output terminal OUT and the output terminal 15, andthe resistor RBm is connected between the output terminal 15 and theground.

When the above-described conditions are established, the divided voltageVd is expressed by a first equation; Vd=(Vo×RBm)/(RAm+RAn+RBm), whereRAm, RBm, and RAn represent values of the resistors RAm, RBm, and RAn,respectively.

The input voltages to the inverse and non-inverse input terminals of theerror amplifier 6, which are Vd and Vr, respectively, are equalized dueto a phenomenon called an imaginary short occurring therebetween. Thatis, in this case, a relationship between Vd and Vr is expressed asVd=Vr. Therefore, the above first equation can be modified as;Vo=Vr×(RAm+RAn+RBm)/RBm.

For another example, when the control signal SPn is raised to the levelH and the control signals SP1-SPn−1 are lowered to the level L, the PMOStransistors QP1-QPn−1 are turned on to be conductive. At the same time,the NMOS transistor QNn is turned on to be conductive and the rest ofthe NMOS transistors QN1-QNn−1 are turned off to be non-conductive.Thereby, the resistor RAn is connected between the output terminal OUTand the output terminal 15, and the resistor RBn is connected betweenthe output terminal 15 and the ground.

Under the above-mentioned conditions, the divided voltage Vd isexpressed by a second equation; Vd=(Vo×RBn)/(RAn+RBn), where RBnrepresents a value of the resistor RBn.

The input voltages to the inverse and non-inverse input terminals of theerror amplifier 6, which are Vd and Vr, respectively, are equalized dueto the imaginary short phenomenon occurring therebetween. That is, inthis case, a relationship between Vd and Vr is expressed as Vd=Vr.Therefore, the above second equation can be modified as;Vo=Vr×(RAn+RBn)/RBn.

In this way, the voltage divider 11 can vary the value of the outputvoltage Vo in accordance with the voltage switching signal Sc. In otherwords, a desirable value of the output voltage Vo can be obtained fromthe voltage divider 11 by suitably selecting the voltage switchingsignal Sc.

FIG. 4 shows an exemplary circuit of the voltage dividing circuit 5 whenn is two. In the voltage dividing circuit 5 of FIG. 4, the resistors RA1and RA2 are connected in series between the output terminal OUT and theoutput terminal 15. The PMOS transistor QP1 is connected parallel to theresistor RA1 and the capacitor 16 is connected between the outputterminal OUT and the output terminal 15.

Further, in the voltage dividing circuit 5 of FIG. 4, the NMOStransistors QN1 and QN2 are coupled to the resistors RB1 and RB2,respectively, to form two sets of transistor-resistor series circuits,which are connected in parallel between the output terminal 15 and theground. The switching controller 12 includes an inverter 17, andgenerates the control signals SP1 and SP2 based on the voltage switchingsignal Sc. The control signal SP1 is a straight signal of the voltageswitching signal Sc and the control signal SP2 is an inverted signal ofthe voltage switching signal Sc. The control signal SP1 is applied tothe gates of the PMOS transistor QP1 and the NMOS transistor QN1, andthe control signal SP2 is applied to the gate of the NMOS transistorQN2.

In the voltage dividing circuit 5 of FIG. 4, the output voltage Vo basedon the above modified first equation can be expressed as;Vo=Vr×(RA1+RA2+RB1)/RB1.

Also, in the voltage dividing circuit 5 of FIG. 4, the output voltage Vobased on the above modified second equation can be expressed as;Vo=Vr×(RA2+RB2)/RB2.

In the power supply apparatus 1 of FIG. 2, the reference voltagegenerator 4, the voltage dividing circuit 5, the error amplifier 6, andthe control circuit 7 are integrated into one IC (integrated circuit)chip. It is possible to further integrate the switching transistor 2into that IC chip.

It is also possible to substitute an NMOS transistor for the flywheeldiode D1. FIG. 5 shows a power supply apparatus 1 a, which is a firstexemplary alternative embodiment based on the power supply apparatus 1of FIG. 2, in which the flywheel diode D1 is substituted by an NMOStransistor 21. In this case, it becomes possible to integrate theswitching transistor 2, the reference voltage generator 4, the voltagedividing circuit 5, the error amplifier 6, the control circuit 7, andthe NMOS transistor 21 into one IC (integrated circuit) chip.

FIG. 6 shows a power supply apparatus 1 b, which is a second exemplaryalternative embodiment based on the power supply apparatus 1 of FIG. 2.The power supply apparatus 1 b of FIG. 6 is an exemplary power supplyapparatus provided with a DC-to-DC (direct current to direct current)converter including a voltage-step-up-type switching regulator.

The power supply apparatus 1 b of FIG. 6 is similar to the power supplyapparatus 1 of FIG. 2, except for a switching regulator 31, a smoothingcircuit 32, and a control circuit 33. The switching regulator 31includes an NMOS transistor and performs a switching operation inaccordance with a control signal input to a gate thereof. The smoothingcircuit 32 smoothes the signal output from the switching transistor 31and outputs a resultant signal having an output voltage Vo to the outputterminal OUT. The control circuit 33 controls the switching operation ofthe switching transistor 31 in accordance with the signal output fromthe error amplifier 6.

The output voltage Vo output to the output terminal OUT is divided bythe voltage dividing circuit 5, and the difference between the resultantdivided voltage Vd, which is regarded as an error, is amplified by theerror amplifier 6. The control circuit 33 includes an oscillator forgenerating, for example, a pulse signal having a triangular waveform anda comparator. In the control circuit 33, the comparator compares thevoltage of the signal output from the oscillator with the voltage of thesignal output from the error amplifier 6. In accordance with a result ofthe comparison, the comparator controls a time that the switchingtransistor 31 is being turned on. The signal output from the switchingtransistor 31 is smoothed and output as the voltage Vo, by the smoothingcircuit 32, which includes a rectifying diode D2, an electric coil L2,and a capacitor C2.

In the power supply apparatus 1 b of FIG. 6, the reference voltagegenerator 4, the voltage dividing circuit 5, the error amplifier 6, andthe control circuit 33 are integrated into one IC (integrated circuit)chip. It is possible to further integrate the switching transistor 31into that IC chip.

FIG. 7 shows a power supply apparatus 1 c, which is a third exemplaryalternative embodiment based on the power supply apparatus 1 of FIG. 2.The power supply apparatus 1 c of FIG. 7 is an exemplary power supplyapparatus provided with a DC-to-DC (direct current to direct current)converter including a series regulator instead of a switching regulator.

The power supply apparatus 1 c of FIG. 7 is similar to the power supplyapparatus 1 of FIG. 2, except for an output control transistor 41 and acapacitor 42. The output control transistor 41 includes a PMOStransistor and outputs to the output terminal OUT a current inaccordance with a voltage input to a gate thereof from the erroramplifier 6. The capacitor 42 stabilizes an output voltage Vo outputthrough the output terminal OUT.

The output voltage Vo output to the output terminal OUT is divided bythe voltage dividing circuit 5, and the difference between the resultantdivided voltage Vd, which is regarded as an error, is amplified by theerror amplifier 6. The error amplifier 6 outputs a resultant voltage toa gate of the output control transistor 41. The error amplifier 6 thuscontrols the operation of the output control transistor 41 so as toregulate the output voltage Vo to a predetermined preferable voltage.

In the power supply apparatus 1 c of FIG. 7, the reference voltagegenerator 4, the voltage dividing circuit 5, and the error amplifier 6are integrated into one IC (integrated circuit) chip. It is possible tofurther integrate the output control transistor 41 into that IC chip.

In the case of the power supply apparatus 1 of FIG. 1, the switchingcontroller 12 of the voltage dividing circuit 5 is configured to causeone of the PMOS transistors QP1-QPn−1 to turn exclusively off or everyone of the PMOS transistors QP1-QPn−1 to turn on and one of the NMOStransistors QN1-QNn to turn exclusively on, in accordance with theexternally input voltage switching signal Sc, as one example. Thissetting of the switching controller 12 in the power supply apparatus 1can also be applied to the switching controller 12 of the power supplyapparatus 1 c, as one example.

As one exemplary alternative, in the power supply apparatus 1 c, theswitching controller 12 of the voltage dividing circuit 5 may cause morethan one of the PMOS transistors QP1-QPn−1 to turn off simultaneouslyand more than one of the NMOS transistors QN1-QNn to turn onsimultaneously, in accordance with the externally input voltageswitching signal Sc.

As another exemplary alternative, in the power supply apparatus 1 c, theswitching controller 12 of the voltage dividing circuit 5 may cause oneof the PMOS transistors QP1-QPn−1 to turn exclusively on or every one ofthe PMOS transistors QP1-QPn−1 to turn off and one of the NMOStransistors QN1-QNn to turn exclusively off, in accordance with theexternally input voltage switching signal Sc.

As described above, in each power supply apparatus 1, 1 a, 1 b, and 1 c,the voltage dividing circuit 5 is configured to have a feedback circuitfor dividing the output voltage Vo to generate the divided voltage Vd.This voltage dividing circuit 5 is further configured to change thevoltage dividing ratio relative to the output voltage Vo in accordancewith the voltage switching signal Sc to change consequently the dividedvoltage Vd. Thereby, the voltage value of the output voltage Vo ischanged in a preferable manner. In other words, each power supplyapparatus 1, 1 a, 1 b, and 1 c has the structure capable of allowing anexternal selection of the output voltage Vo from a plurality ofpredetermined voltage values. Therefore, any one power supply apparatus1, 1 a, 1 b, and 1 c can comply with a change in the power requirementsof the load circuit by easily changing the output voltage value withoutthe needs of changing the power supply apparatus itself.

In addition, in any one power supply apparatus 1, 1 a 1 b, and 1 c, eachof the resistors RB1-RBn coupled to the NMOS transistors QN1-QNn,respectively, is turned into conductive status when corresponding one ofthe NMOS transistors QN1-QNn is turned off into non-conductive status.That is, a parasitic capacitor of each resistor can be disregarded inthe status that the NMOS transistor is out of conduction. This statusfacilitates a phase design of the voltage dividing circuit 5.

Referring to FIG. 8, a power supply apparatus 51 according to anotherpreferred embodiment of the present invention is explained. FIG. 8 showsthe power supply apparatus 51 which is similar to the power supplyapparatus 1 of FIG. 2, having a DC-to-DC (direct current to directcurrent) converter including a voltage-step-down-type switchingregulator, except for a voltage dividing circuit 52. As shown in FIG. 8,the power supply apparatus 51 is connected to the direct current powersource 10, and includes the switching transistor 2, the smoothingcircuit 3, the reference voltage generator 4, the voltage dividingcircuit 52, the error amplifier 6, and the control circuit 7.

The voltage dividing circuit 52 includes a voltage divider 61 and aswitching controller 62. The voltage divider 61 switches a dividingratio in accordance with an input control signal, and accordinglygenerates and outputs the divided voltage Vd. The switching controller62 controls the switching of the dividing ratio of the voltage divider61 in accordance with a voltage switching signal Sc which is externallyinput thereto.

FIG. 9 breaks down the voltage divider 61 of the voltage dividingcircuit 52 and, with reference thereto, an exemplary operation of thevoltage dividing circuit 52 is explained. The voltage divider 61includes n resistors referred to as resistors RC1-RCn, n resistors inseries referred to as resistors RD1-RDn, n PMOS transistors referred toas PMOS transistors QP1-QPn, (n−1) N-channel MOS transistors referred toas NMOS transistors QN1-QNn−1, and a capacitor 16. The resistors RC1-RCnare configured as a first resistor circuit, the resistors RD1-RDn areconfigured as a second resistor circuit, the PMOS transistors QP1-QPnare configured as a first switching circuit, and the NMOS transistorsQN1-QNn−1 are configured as a second switching circuit.

The resistors RD1-RDn are connected in series between an output terminal65 of the voltage dividing circuit 61 and the ground. The NMOStransistors QN1-QNn−1 are coupled to the resistors RD1-RDn−1,respectively, to form n−1 sets of transistor-resistors parallelcircuits, which are connected in series between the output terminal 65and the ground. The switching controller 62 generates control signalsSN1-SNn based on the voltage switching signal Sc, and applies thecontrol signals SN1-SNn−1 to gates of the NMOS transistors QN1-QNn−1,respectively.

The capacitor 16 is also connected between the output terminal OUT ofthe power supply apparatus 51 and the output terminal 65. The PMOStransistors QP1-QPn are coupled to the resistors RC1-RCn, respectively,to form n sets of transistor-resistor series circuits, which areconnected in parallel between the output terminal OUT of the powersupply apparatus 51 and the ground. The control signals SN1-SNn of theswitching controller 62 are also applied to gates of the PMOStransistors QP1-QPn, respectively.

The switching controller 62 drops one of the control signals SN1-SNndown to the level L and raises the rest of the control signals to thelevel H in accordance with the externally input voltage switching signalSc. For example, when the control signal SNm is dropped down to thelevel L and the rest of the control signals are raised to the level H,the PMOS transistor QPm is selected among the PMOS transistors QP1-QPnto be turned on to be conductive and the rest of the PMOS transistorsare turned on to be non-conductive, where m is an integer between 1 andn−1. At the same time, the NMOS transistor QNm is selected among theNMOS transistors QN1-QNn−1 to be turned off to be non-conductive and therest of the NMOS transistors are turned on to be conductive. Thereby,the resistor RCm is connected between the output terminal OUT and theoutput terminal 65, and the resistors RDm and RDn are connected inseries between the output terminal 65 and the ground.

When the above-described conditions are established, the divided voltageVd is expressed by a third equation; Vd=Vo×(RDm+RDn)/(RCm+RDm+RDn),where RCm, RDm, and RDn represent values of the resistors RCm, RDm, andRDn, respectively.

The input voltages to the inverse and non-inverse input terminals of theerror amplifier 6, which are Vd and Vr, respectively, are equalized dueto the imaginary short phenomenon occurring therebetween. That is, inthis case, a relationship between Vd and Vr is expressed as Vd=Vr.Therefore, the above third equation can be modified as;Vo=Vr×(RCm+RDm+RDn)/(RDm+RDn).

As another example, when the control signal SNn is dropped down to thelevel L and the control signals SN1-SNn−1 are raised to the level H, thePMOS transistors QP1-QPn−1 are turned off to be non-conductive. At thesame time, the PMOS transistor QPn is turned on to be conductive and theNMOS transistors QN1-QNn−1 are turned on to be conductive. Thereby, theresistor RCn is connected between the output terminal OUT and the outputterminal 65, and the resistor RDn is connected between the outputterminal 65 and the ground.

Under the above-mentioned conditions, the divided voltage Vd isexpressed by a fourth equation; Vd=(Vo×RDn)/(RCn+RDn), where RCnrepresents a value of the resistor RCn.

The input voltages to the inverse and non-inverse input terminals of theerror amplifier 6, which are Vd and Vr, respectively, are equalized dueto the imaginary short phenomenon occurring therebetween. That is, inthis case, a relationship between Vd and Vr is expressed as Vd=Vr.Therefore, the above fourth equation can be modified as;Vo=Vr×(RCn+RDn)/RDn.

In this way, the voltage divider 51 can vary the value of the outputvoltage Vo in accordance with the voltage switching signal Sc. In otherwords, a desirable value of the output voltage Vo can be obtained fromthe voltage divider 51 by suitably selecting the voltage switchingsignal Sc.

FIG. 10 shows an exemplary circuit of the voltage dividing circuit 52when n is two. In the voltage dividing circuit 52 of FIG. 10, the PMOStransistors QP1 and QP2 are coupled to the resistors RC1 and RC2,respectively, to form two pieces of transistor-resistor series circuitswhich are connected in parallel between the output terminal OUT and theoutput terminal 65. Also, the capacitor 16 is connected between theoutput terminal OUT and the output terminal 65.

Further, the resistors RD1 and RD2 are connected between the outputterminal 65 and the ground, and the NMOS transistor QN1 is coupled tothe resistor RD1 to form a transistor-resistor series circuit which isconnected in series to the resistor RD2 between the output terminal 65and the ground. The switching controller 62 includes an inverter 67, andgenerates the control signals SN1 and SN2 based on the voltage switchingsignal Sc. The control signal SN1 is a straight signal of the voltageswitching signal Sc as it is and the control signal SN2 is an invertedsignal of the voltage switching signal Sc. The control signal SN1 isapplied to the gates of the PMOS transistor QP1 and the NMOS transistorQN1, and the control signal SN2 is applied to the gate of the PMOStransistor QP2.

In the voltage dividing circuit 52 of FIG. 10, the output voltage Vobased on the above modified third equation can be expressed as;Vo=Vr×(RC1+RD1+RD2)/(RD1+RD2).

Also, the above modified fourth equation can be expressed as;Vo=Vr×(RC2+RD2)/RD2.

In the power supply apparatus 1 of FIG. 8, the reference voltagegenerator 4, the voltage dividing circuit 52, the error amplifier 6, andthe control circuit 7 are integrated into one IC (integrated circuit)chip. It is possible to further integrate the switching transistor 2into that IC chip.

It is also possible to substitute an NMOS transistor for the flywheeldiode D1. FIG. 5 shows a power supply apparatus 1 a which is a firstmodified version based on the power supply apparatus 1 of FIG. 2, inwhich the flywheel diode D1 is substituted by an NMOS transistor, as inthe case of FIG. 5. In this case, it becomes possible to integrate theswitching transistor 2, the reference voltage generator 4, the voltagedividing circuit 52, the error amplifier 6, the control circuit 7, andthe NMOS transistor into one IC (integrated circuit) chip.

To use a DC-to-DC (direct current to direct current) converter includinga voltage-step-up-type switching regulator as an alternative to theDC-to-DC converter including the voltage-step-down-type switchingregulator, the power supply apparatus 51 of FIG. 8 may become the onesimilar to the power supply apparatus 1 b shown in FIG. 6, except thatthe voltage dividing circuit 5 is replaced with the voltage dividingcircuit 52. In such a power supply apparatus, the reference voltagegenerator 4, the voltage dividing circuit 52, the error amplifier 6, andthe control circuit 33 can be integrated into one IC (integratedcircuit) chip. It is also possible to further integrate the switchingtransistor 31 into that IC chip.

Further, to use a series regulator instead of a switching regulator, thepower supply apparatus 51 of FIG. 8 may become the one similar to thepower supply apparatus 1 c shown in FIG. 7, except that the voltagedividing circuit 5 is replaced with the voltage dividing circuit 52. Insuch a power supply apparatus, the reference voltage generator 4, thevoltage dividing circuit 52, and the error amplifier 6 can be integratedinto one IC (integrated circuit) chip. It is also possible to furtherintegrate the output control transistor 41 into that IC chip.

In the power supply apparatus 51 of FIG. 8, the switching controller 62of the voltage dividing circuit 52 causes one of the PMOS transistorsQP1-QPn to turn exclusively on and one of the NMOS transistors QN1-QNn−1to turn exclusively off or every one of the NMOS transistors QN1-QNn−1to turn on, in accordance with the externally input voltage switchingsignal Sc, as one example.

As an alternative, the switching controller 62 may cause more than oneof the PMOS transistors QP1-QPn to turn on simultaneously and more thanone of the NMOS transistors QN1-QNn−1 to turn off simultaneously, inaccordance with the externally input voltage switching signal Sc.

As another alternative, the switching controller 12 may cause one of thePMOS transistors QP1-QPn to turn exclusively off and one of the NMOStransistors QN1-QNn−1 to turn exclusively on or every one of the NMOStransistors QN1-QNn−1 to turn off, in accordance with the externallyinput voltage switching signal Sc.

As described above, in the power supply apparatus 51, the voltagedividing circuit 52 is configured to have a feedback circuit fordividing the output voltage Vo to generate the divided voltage Vd. Thisvoltage dividing circuit 52 is further configured to change the voltagedividing ratio relative to the output voltage Vo in accordance with thevoltage switching signal Sc to change consequently the divided voltageVd. Thereby, the voltage value of the output voltage Vo is changed in apreferable manner. In other words, each one of the power supplyapparatus 51 has the structure capable of allowing an external selectionof the output voltage Vo from a plurality of predetermined voltagevalues. Therefore, the power supply apparatus 51 can comply with achange in the power requirements of the load circuit by easily changingthe output voltage value without the needs of changing the power supplyapparatus itself.

While the invention has been described and illustrated with reference tospecific exemplary embodiments, it should be understood that manymodifications and substitutions can be made without departing from thespirit and scope of the invention. Accordingly, the invention is not tobe considered as limited by the foregoing description but is onlylimited by the scope of the appended claims.

1. A power supply apparatus, comprising: an input terminal having aninput voltage applied thereto from a direct current power source; anoutput voltage generator configured to generate a constant outputvoltage based on said input voltage; an output terminal of said outputvoltage generator outputting said constant output voltage; a referencevoltage generator configured to generate a reference voltage; a voltagedivider having an output point, said voltage divider configured toaccept said constant output voltage from said output terminal of saidoutput voltage generator and further configured to divide said constantoutput voltage into a divided voltage in accordance with a voltagedividing ratio which is variable in response to an externally-inputcontrol signal and to output said divided voltage to said output pointof said voltage divider, wherein said voltage divider comprises a firstresistance circuit including a plurality of resistors connectable inseries, and a second resistance circuit connected in series with saidfirst resistance circuit, said second resistance circuit including aplurality of resistors connectable in parallel including at least oneswitchably controllable resistor; and a voltage control circuitconfigured to control said output voltage generator to regulate saidconstant output voltage such that said divided voltage from said voltagedivider is equalized to said reference voltage.
 2. The power supplyapparatus as defined in claim 1, wherein: said first resistance circuitis connected in series between said output terminal and said outputpoint of said voltage divider; a first switch circuit configured to makea short circuit in at least one of said plurality of resistors includedin said first resistance circuit in response to an input control signal;a second switch circuit configured to connect at least one of saidplurality of resistors included in said second resistance circuitbetween said output point of said voltage divider and a common ground ofsaid direct current power source in response to said input controlsignal; and a switch control circuit configured to generate said inputcontrol signal in response to said externally-input control signal andto control said first and second switch circuits with said input controlsignal to change the voltage dividing ratio.
 3. The power supplyapparatus as defined in claim 1, wherein said voltage divider furthercomprises: a first switch circuit configured to connect in parallel withat least one of said plurality of resistors included in said secondresistance circuit between said output terminal and said output point ofsaid voltage divider in response to an input control signal; and whereinsaid first resistance circuit is connected in series between said outputpoint of said voltage divider and a common ground of said direct currentpower source; a second switch circuit configured to make a short circuitin at least one of said plurality of resistors included in said firstresistance circuit in response to said input control signal; and aswitch control circuit configured to generate said input control signalin response to said externally-input control signal and to control saidfirst and second switch circuits with said input control signal tochange the voltage dividing ratio.
 4. The power supply apparatus asdefined in claim 1, wherein said output voltage generator includes aswitching transistor performing a switching operation for outputting theinput voltage applied by the direct current power source in accordancewith a control signal from said voltage control circuit, wherein saidvoltage control circuit comprises: an error amplifier amplifying anerror of said divided voltage output from said output point of saidvoltage divider relative to said reference voltage; a control circuitconfigured to generate said control signal in accordance with an outputsignal from said error amplifier to control said switching operation ofsaid switching transistor; and a smoothing circuit configured to smoothan output signal from said switching transistor and to output saidsmoothed output signal to said output terminal.
 5. The power supplyapparatus as defined in claim 4, wherein said reference voltagegenerator, said voltage divider, said error amplifier, and said controlcircuit are integrated into a single integrated circuit.
 6. The powersupply apparatus as defined in claim 4, wherein said reference voltagegenerator, said switching transistor, said voltage divider, said erroramplifier, and said control circuit are integrated into a singleintegrated circuit.
 7. A power supply apparatus, comprising: an inputterminal having an input voltage applied thereto from a direct currentpower source; an output voltage generator configured to generate aconstant output voltage based on said input voltage; an output terminalof said output voltage generator outputting said constant outputvoltage; a reference voltage generator configured to generate areference voltage; a voltage divider having an output point, saidvoltage divider configured to accept said constant output voltage fromsaid output terminal of said output voltage generator and furtherconfigured to divide said constant output voltage into a divided voltagein accordance with a voltage dividing ratio which is variable inresponse to an externally-input control signal and to output saiddivided voltage to said output point of said voltage divider; a voltagecontrol circuit configured to control said output voltage generator toregulate said constant output voltage such that said divided voltagefrom said voltage divider is equalized to said reference voltage; saidoutput voltage generator includes a switching transistor performing aswitching operation for outputting the input voltage applied by thedirect current power source in accordance with a control signal fromsaid voltage control circuit, wherein said voltage control circuitcomprises: an error amplifier amplifying an error of said dividedvoltage output from said output point of said voltage divider relativeto said reference voltage, a control circuit configured to generate saidcontrol signal in accordance with an output signal from said erroramplifier to control said switching operation of said switchingtransistor, and a smoothing circuit configured to smooth an outputsignal from said switching transistor and to output said smoothed outputsignal to said output terminal; said reference voltage generator, saidvoltage divider, said error amplifier, and said control circuit areintegrated into a single integrated circuit; said reference voltagegenerator, said switching transistor, said voltage divider, said erroramplifier, and said control circuit are integrated into a singleintegrated circuit; and wherein said smoothing circuit includes atransistor which is operated and controlled by said control circuit tofunction as a flywheel diode, and said transistor, said switchingtransistor, said voltage divider, said switching transistor, said erroramplifier, and said control circuit are integrated into a singleintegrated circuit.
 8. The power supply apparatus as defined in claim 1,wherein said output voltage generator includes an output controltransistor controlling an output of a current applied by the directcurrent power source in accordance with a control signal from saidvoltage control circuit, wherein said voltage control circuit comprisesan error amplifier controlling an operation of said output controltransistor such that said divided voltage of said voltage divider isequalized to said reference voltage.
 9. The power supply apparatus asdefined in claim 8, wherein said reference voltage generator, saidvoltage divider, and said error amplifier are integrated into a singleintegrated circuit.
 10. The power supply apparatus as defined in claim8, wherein said reference voltage generator, said voltage divider, saiderror amplifier, and said output control transistor are integrated intoa single integrated circuit.
 11. A power supply apparatus, comprising:input terminal means for having an input voltage applied thereto from adirect current power source; output voltage generating means forgenerating a constant output voltage based on said input voltage; outputterminal means for outputting said constant output voltage; referencevoltage generating means for generating a reference voltage; voltagedividing means for receiving said constant output voltage from saidoutput terminal means and for dividing said constant output voltage intoa divided voltage in accordance with a voltage dividing ratio which isvariable in response to an externally-input control signal and foroutputting said divided voltage, wherein said voltage dividing meanscomprises a first resistance circuit including a plurality of resistorsconnectable in series, and a second resistance circuit connected inseries with said first resistance circuit, said second resistancecircuit including a plurality of resistors connectable in parallelincluding at least one switchably controllable resistor;and voltagecontrolling means for controlling said output voltage generating meansto regulate said constant output voltage such that said divided voltagefrom said voltage dividing means is equalized to said reference voltage.12. The power supply apparatus as defined in claim 11, wherein: saidfirst resistance circuit is connected in series between said outputterminal means and said output point of said voltage dividing means; afirst switch circuit configured to make a short circuit in at least oneof said plurality of resistors included in said first resistance circuitin response to an input control signal; a second switch circuitconfigured to connect at least one of said plurality of resistorsincluded in said second resistance circuit between said output point ofsaid voltage dividing means and a common ground of said direct currentpower source in response to said input control signal; and a switchcontrol circuit configured to generate said input control signal inresponse to said externally-input control signal and to control saidfirst and second switch circuits with said input control signal tochange the voltage dividing ratio.
 13. The power supply apparatus asdefined in claim 11, wherein said voltage dividing means furthercomprises: a first switch circuit configured to connect in parallel withat least one of said plurality of resistors included in said secondresistance circuit between said output terminal and said output point ofsaid voltage dividing means in response to an input control signal; andwherein said first resistance circuit is connected in series betweensaid output point of said voltage dividing means and a common ground ofsaid direct current power source; a second switch circuit configured tomake a short circuit in at least one of said plurality of resistorsincluded in said first resistance circuit in response to said inputcontrol signal; and a switch control circuit configured to generate saidinput control signal in response to said externally-input control signaland to control said first and second switch circuits with said inputcontrol signal to change the voltage dividing ratio.
 14. The powersupply apparatus as defined in claim 11, wherein said output voltagegenerating means includes switching means for performing a switchingoperation for outputting the input voltage applied by the direct currentpower source in accordance with a control signal from said voltagecontrolling means, and said voltage controlling means comprises: erroramplifying means for amplifying an error of said divided voltage outputfrom said output point of said voltage dividing means relative to saidreference voltage; controlling means for generating said control signalin accordance with an output signal from said error amplifying means tocontrol said switching operation of said switching means; and smoothingmeans for smoothing an output signal from said switching means and tooutput said output signal to said output terminal means.
 15. The powersupply apparatus as defined in claim 14, wherein said reference voltagegenerating means, said voltage dividing means, said error amplifyingmeans, and said controlling means are integrated into a singleintegrated circuit.
 16. The power supply apparatus as defined in claim14, wherein said reference voltage generating means, said switchingmeans, said voltage dividing means, said error amplifying means, andsaid controlling means are integrated into a single integrated circuit.17. A power supply apparatus, comprising: input terminal means forhaving an input voltage applied thereto from a direct current powersource; output voltage generating means for generating a constant outputvoltage based on said input voltage; output terminal means foroutputting said constant output voltage; reference voltage generatingmeans for generating a reference voltage; voltage dividing means forreceiving said constant output voltage from said output terminal meansand for dividing said constant output voltage into a divided voltage inaccordance with a voltage dividing ratio which is variable in responseto an externally-input control signal and for outputting said dividedvoltage; and voltage controlling means for controlling said outputvoltage generating means to regulate said constant output voltage suchthat said divided voltage from said voltage dividing means is equalizedto said reference voltage; said output voltage generating means includesswitching means for performing a switching operation for outputting theinput voltage applied by the direct current power source in accordancewith a control signal from said voltage controlling means, and saidvoltage controlling means comprises: error amplifying means foramplifying an error of said divided voltage output from said outputpoint of said voltage dividing means relative to said reference voltage,controlling means for generating said control signal in accordance withan output signal from said error amplifying means to control saidswitching operation of said switching means, and smoothing means forsmoothing an output signal from said switching means and to output saidoutput signal to said output terminal means; said reference voltagegenerating means, said voltage dividing means, said error amplifyingmeans, and said controlling means are integrated into a singleintegrated circuit; said reference voltage generating means, saidswitching means, said voltage dividing means, said error amplifyingmeans, and said controlling means are integrated into a singleintegrated circuit; and wherein said smoothing means includes atransistor which is operated and controlled by said controlling means tofunction as a flywheel diode, and said transistor, said switching means,said voltage dividing means, said smoothing means, said error amplifyingmeans, and said controlling means are integrated into a singleintegrated circuit.
 18. The power supply apparatus as defined in claim11, wherein said output voltage generating means includes an outputcontrol transistor controlling an output of a current applied by thedirect current power source in accordance with a control signal fromsaid voltage controlling means, and said voltage controlling meanscomprises error amplifying means for controlling an operation of saidoutput control transistor such that said divided voltage of said voltagedividing means is equalized to said reference voltage.
 19. The powersupply apparatus as defined in claim 18, wherein said reference voltagegenerating means, said voltage dividing means, and said error amplifyingmeans are integrated into a single integrated circuit.
 20. The powersupply apparatus as defined in claim 18, wherein said reference voltagegenerating means, said voltage dividing means, said error amplifyingmeans, and said output control transistor are integrated into a singleintegrated circuit.
 21. A power supply apparatus, comprising: an inputvoltage from a direct current power source applied to an input terminal;a reference voltage generator for generating a reference voltage; anoutput control transistor for outputting a constant output voltage to anoutput terminal in accordance with a voltage input to a gate of saidoutput control transistor; and a voltage divider having an output point,said voltage divider configured to accept said constant output voltagefrom said output terminal of said output voltage generator and furtherconfigured to divide said constant output voltage into a divided voltagein accordance with a voltage dividing ratio which is variable inresponse to an external-input control signal and output said dividedvoltage to said output point of said voltage divider, wherein saidvoltage divider comprises a first resistance circuit including aplurality of resistors connectable in series, and a second resistancecircuit connected in series with said first resistance circuit, saidsecond resistance circuit including a plurality of resistors connectablein parallel including at least one switchably controllable resistor. 22.The power supply apparatus as defined in claim 21, further comprising anerror amplifier amplifying an error between said divided voltage outputfrom said output point of said voltage divider and said referencevoltage.
 23. The power supply apparatus as defined in claim 21, furthercomprising a capacitor for stabilizing said constant output voltagethrough said output terminal, said capacitor coupled to ground andfurther coupled to said constant output voltage via said output controltransistor.
 24. A method of operating a power supply apparatuscomprising: applying an input voltage from a direct current powersource; outputting a constant output voltage based on said inputvoltage; generating a reference voltage; dividing said constant outputvoltage in accordance with a voltage dividing ratio in response to anexternal-input control signal to produce a divided voltage, wherein saidvoltage dividing ratio is obtained using a first resistance circuitincluding a plurality of resistors connectable in series, and a secondresistance circuit connected in series with said first resistancecircuit, said second resistance circuit including a plurality ofresistors connectable in parallel including at least one switchablycontrollable resistor; and regulating said constant output voltage suchthat said divided voltage is equalized to said reference voltage. 25.The method as defined in claim 24, further comprising: amplifying anerror between said divided voltage and said reference voltage; andgenerating a control signal based on said amplified error, said controlsignal controlling said act of regulating.
 26. The method as defined inclaim 25, further comprising smoothing said regulated constant outputvoltage.